Vehicle fuel system storage tanks inevitably develop elevated tank pressures as liquid fuel vaporizes into the space between the surface of the fuel and the top of the tank. The vapor so formed is actually a mixture of air and fuel vapors, which forms mainly due to fuel heating. When the tank pressure rises above what is considered a safe threshold, it must be vented somehow. Old systems simply had a vent pipe to atmosphere that was always open. Currently, in order to reduce the emission of raw hydrocarbons into the air, vehicles vent the tank instead to an activated charcoal storage canister. The fuel vapor component of the mixture is adsorbed on the surface of the charcoal granules, so that only the air is expelled to atmosphere. Later, manifold vacuum from the engine is used to draw air from atmosphere back through the canister, purging the fuel vapor to be burned in the engine. Such systems have worked well, but more stringent emission regulations in the future may require even larger canisters to store even more fuel vapor. The prospect of having to burn even more purged fuel vapor has raised concerns about potentially raising exhaust emissions.